Among known techniques for introducing an impurity into a surface layer of a solid sample is a plasma doping method in which an impurity is ionized and introduced into a solid at low energy (refer to Patent document 1, for example).
FIG. 12 shows a general configuration of a plasma processing apparatus which is used for a plasma doping method as a conventional impurity introducing method disclosed in the above-mentioned Patent document 1. As shown in FIG. 12, a sample electrode 43 to be mounted with a sample 42 which is a silicon wafer is provided in a vacuum container 41. A gas supply apparatus 44 for supplying a doping source gas containing a desired element such as B2H6 to the inside of the vacuum container 41 and a pump 45 for reducing the pressure in the vacuum container 41 are provided, whereby the pressure in the vacuum container 41 can be kept at a prescribed value. Microwaves are radiated from a microwave waveguide 46 into the vacuum container 41 via a quartz plate 47 as a dielectric window. The microwaves interact with a DC magnetic field formed by an electromagnet 48, whereby microwave plasma with a magnetic field (electron cyclotron resonance plasma) 49 is formed in the vacuum container 41. A high-frequency power source 51 is connected to the sample electrode 43 via a capacitor 50 so as to enable control of the potential of the sample electrode 43. A gas that is supplied form the gas supply apparatus 44 is introduced into the vacuum container 41 through a gas inlet 52 and exhausted into the pump 45 through an exhaust hole 53.
In the above-configured plasma processing apparatus, a doping source gas such as B2H6 that is introduced through the gas inlet 52 is converted into plasma 49 by a plasma generating means consisting of the microwave waveguide 46 and the electromagnet 48 and boron ions in the plasma 49 are supplied to the surface of a sample 42 by means of the high-frequency power source 51.
Incidentally, in general, gases containing an impurity that is rendered electrically active when supplied to a sample such as a silicon wafer, such as a doping source gas B2H6, have a problem that they are very dangerous; for example, they are harmful to human bodies or high in reactivity.
In plasma doping methods, all substances contained in a doping source gas are introduced into a sample. In the case of a doping source gas B2H6, for example, hydrogen is also introduced into a sample though only boron is an impurity that is effective when introduced into the sample.
Introduction of hydrogen into a sample raises a problem that lattice defects occur in the sample in subsequent heat treatment for epitaxial growth, for example.
In view of the above, the following method has been proposed (refer to Patent document 2, for example). An impurity solid containing an impurity which is rendered electrically active when introduced into a sample is disposed in a vacuum container. Plasma of a rare gas is generated in the vacuum container and an impurity solid is bombarded with ions of the inert gas, whereby the impurity is separated from the impurity solid and supplied to the sample. FIG. 13 shows a general configuration of a plasma doping apparatus which is used in a plasma doping method as the conventional impurity introducing method disclosed in Patent document 2. As shown in FIG. 13, a sample electrode 43 to be mounted with a sample 42 which is a silicon wafer is provided in a vacuum container 41. A gas supply apparatus 44 for supplying an inert gas to the inside of the vacuum container 41 and a pump 45 for reducing the pressure in the vacuum container 41 are provided, whereby the pressure in the vacuum container 41 can be kept at a prescribed value. Microwaves are radiated from a microwave waveguide 46 into the vacuum container 41 via a quartz plate 47 as a dielectric window. The microwaves interact with a DC magnetic field formed by an electromagnet 48, whereby microwave plasma with a magnetic field (electron cyclotron resonance plasma) 49 is formed in the vacuum container 41. A high-frequency power source 51 is connected to the sample electrode 43 via a capacitor 50, whereby the potential of the sample electrode 43 can be controlled. An impurity solid 54 containing an impurity element such as boron is mounted on a solid holding stage 55, whose potential is controlled by a high-frequency power source 57 connected to it via a capacitor 56. A gas that is supplied form the gas supply apparatus 44 is introduced into the vacuum container 41 through a gas inlet 52 and exhausted into the pump 45 through an exhaust hole 53.
In the above-configured plasma doping apparatus, an inert gas such as argon (Ar) introduced through the gas inlet 11 is converted into plasma by a plasma generating means consisting of the microwave waveguide 46 and the electromagnet 48, and part of impurity element atoms that have been expelled from the impurity solid 54 by the bombardment into the plasma are ionized and introduced into a surface layer of a sample 42.
Usually, a silicon oxide film as a gate oxide film is formed on the surface of the sample 42. A conductive layer for formation of gate electrodes is formed on the gate oxide film by CVD or the like and then patterned into gate electrode patterns. The sample 42 on which the gate electrodes have been formed in this manner is set in the plasma doping apparatus, and source and drain regions are formed by introducing an impurity by the above-described method in a self-aligned manner using the gate electrodes as a mask. MOS transistors are thus obtained. However, activation processing needs to be performed after the introduction of the impurity by the plasma doping. The activation processing is processing of rendering the crystal in an active state by heating the sample 42 by flash lamp annealing, laser annealing, or the like. A shallow activation layer can be obtained by heating a very thin impurity-introduced layer effectively. To heat a very thin impurity-introduced layer effectively, processing for increasing, before introduction of the impurity, the absorbance, for light emitted from a light source such as a laser or a lamp, of a very thin layer into which to introduce the impurity is performed. This processing, which is called “pre-amorphyzation,” is as follows. In a plasma processing apparatus which is similar in configuration to the above-described plasma doping apparatus, plasma of a He gas, for example, is generated and generated He ions, for example, are caused to be accelerated toward and collide with a substrate by a bias voltage, whereby the crystal structure of a substrate surface layer is destroyed to attain amorphyzation. This technique has already been proposed by the inventors of this application (refer to Non-patent document 1, for example).
Patent document 1: U.S. Pat. No. 4,912,065
Patent document 2: JP-A-09-115851
Non-patent document 1: Y. Sasaki et al., “B2H6 Plasma Doping with In-situ He Pre-amorphyzation,” 2004 Symposia on VLSI Technology and Circuits.